Semiconductor substrates are fabricated using low pressure chemical vapor deposition (LPCVD) processes. During this process, an LPCVD chamber is used to contain semiconductor substrates and certain chemicals at high temperatures. The LPCVD chamber is heated up using a heat producing lamp or coil furnace as is known in the art. During the LPCVD processes, the LPCVD chamber is evacuated to form a vacuum therein at a pressure roughly at 1 mTorr.
FIG. 1 illustrates a prior art arrangement for an LPCVD process. The LPCVD chamber 10 in this example is a quartz tube. Chamber 10 contains the semiconductor substrates 15 (also called "wafers"). A quartz ball socket 14a is coupled to chamber 10 using a quartz neck 12. The end of the quartz tube containing the neck 12 and the ball socket 14a is called the "back" end. A low pressure vacuum system 20 (e.g., 760 Torr to 1 m Toro is coupled to a stainless steel vacuum line 18 (pumping line) which is coupled to a stainless steel ball cover 14b. The ball cover 14b and the quartz ball socket 14a comprise the ball junction 14 of FIG. 1 which is used as an interface between the LPCVD chamber 10 and the vacuum system 20.
FIG. 2 illustrates the prior art ball junction arrangement of FIG. 1. This ball junction uses an elastomer O-ring gasket. An O-ring gasket 30 is inserted into the ball junction between the quartz ball junction 14a and the metal cover 14b to provide a seal for the prior art ball junction.
FIG. 3A is an illustration of the metal ball cover 14a looking down direction 25 of FIG. 2. FIG. 3B is a cross section illustration of the metal ball cover 14a. With reference to FIG. 3A and FIG. 3B, a thin receiving groove 23 is cut within the metal ball cover 14a for receiving the O-ring gasket 30. In this way, the O-ring gasket 30 provides a seal between the metal ball cover 14b and the quartz ball socket 14a.
A problem with this prior art design is that the O-ring elastomer 30 becomes denatured at temperatures near 180 degrees Celsius and above. However, the temperature-of the chamber 10 during LPCVD processing is typically maintained at 800 degrees Celsius (e.g., at the end near the heat lamps). A temperature gradient is established wherein the temperature at the back end of the quartz tube (e.g., near the neck 12 and the ball socket 14a) is maintained at around 140 degrees Celsius so as to not denature the O-ring socket whereas the other end of the tube is much hotter.
At slow pump, when the heated contents of the LPCVD chamber 10 are pumped out through the prior art ball junction, it is very difficult to precisely maintain the back-end temperature to 180 degrees Celsius. During slow pump, heat conducts as the tube pumps down during high gas load pumping from 760 Torr to 2 Torr. The neck 12 and ball socket 14a junction will continue to elevate in temperature determined by time and pressure, causing the O-ring 30 to fail (e.g., denature) which causes vacuum leaks. Therefore, maintaining the back end temperature to 180 degrees is very difficult during slow pump, and if not done precisely, causes the O-ring 30 to denature.
Another problem with the above prior art design is that chemicals (e.g., ammonium chloride) used for LPCVD processes tend to condense around the ball junction if the back end temperature is not maintained high enough (e.g., above 140 Degrees Celsius). Resublimation of ammonium chloride create unwanted haze and particulates that build up within the chamber and will reduce yield. The prior art requires a delicate temperature balance at the ball junction where temperatures above 180 degrees Celsius cause O-ring denaturing while temperatures below 140 degrees Celsius cause unwanted chemical condensation. It would be advantageous to eliminate this required delicate temperature regulation, avoid the chemical condensation problem, and also avoid the O-ring denaturing problem. The present invention offers such a ball junction seal.
Accordingly, the present invention provides a ball junction that does not require delicate back end temperature regulation and thus decreases the complexity of the overall LPCVD process. Using the present invention, the back end of the LPCVD chamber can be operated at a temperature of 800 degrees Celsius during slow pump without denaturing any seal. At this temperature, chemical condensation is not a problem and yields improve. These and other advantages of the present invention will become apparent within discussions of the present invention herein.